FIG. 1 shows schematically the configuration of a typical cellular radio telecommunications network. The network comprises a number of base-stations (BSs) 1, 2, 3 etc. Each base-station has a radio transceiver capable of transmitting radio signals to and receiving radio signals from the area of a cell 4, 5 etc. next to the base-station. By means of these signals the base-station can communicate with a mobile station (MS) 6 in that cell, which itself includes a radio transceiver. Each base station is connected to a mobile system controller (MSC) 7, which is linked in turn to the public telephone network 8. By means of this system a user of the MS 6 can establish a telephone call to the public network 8 via the BS in whose cell the MS is located.
The location of the MS could be fixed (for example if it is providing radio communications for a fixed building) or the MS could be moveable (for example if it is a hand portable transceiver or “mobile phone”). When the MS is moveable it may move between cells of the cellular radio system. As it moves from one cell (the “old cell”) to another cell (the “new cell”) there is a need to hand it over from communication with the BS of the old cell to the BS of the new cell without dropping the call due to a break in communications between the mobile station and the network. This process is known as handover. A need can also arise to hand over a MS whose location is fixed, for example if atmospheric conditions affect its communications with the old BS and call quality can be improved by handing it over to another BS or if there is a need to free up capacity of the old BS.
In a conventional cellular radio system handover is controlled automatically by the MSC. Handover can be initiated by the MS or be network dependent, for example, on the quality of the signalling between the MS and the old and new BSs.
When a new cellular network is being introduced it can take some time to install all the base-stations and associated apparatus. Therefore, there is a delay before the new network provides full geographical coverage. FIG. 2 illustrates the situation: an existing cellular network provides full geographical coverage by means of cells 20-27 but the new cellular network provides incomplete geographical coverage by means of only cells 28 and 29. This presents a significant commercial problem for the operator of the new network. If the new network is launched for use before its geographical coverage is complete then customers will be dissatisfied by its inferior coverage to the old network. However, the cost of the infrastructure of the new network is high and no return can be gained on it until it is in use.
It has been proposed to tackle this problem by allowing mobile stations using the new network to be handed over to cells of the old network when they move outside the coverage of the new network. For instance, when a mobile station moves from 30 to 31 in FIG. 2 it could be handed over from the base station of cell 28 (in the new network) to that of cell 21 (in the old network). However, the base-stations of cells 28 and 21 are in different networks and are therefore not linked by a common MSC, so conventional handover processes cannot be used. One solution to this could be to modify the old network to allow it to support inter-network handover. However, modifying the old network would be expensive and inconvenient.
There is therefore a need for a new method for handover between two telecommunications networks.